I made it by soaking crochet string in fabric starch and then wrapping it around a balloon. Then you wait a day or two till its dry, pop the balloon, cut a hole in it, decorate and smash the bottom in a little so it sits flat.

I actually used a corn starch and water mix since the only fabric starch I could find was the kind in a spray can.

They're not at all waterproof. But as long as you don't get it wet then the shape should last for good, assuming it doesn't get crushed or anything like that. They seen to be about as strong as a thin wicker basket